The present invention relates to power supply circuits. More particularly, the present invention relates to a low drop-out regulator having composite amplifier configured to provide a higher performance power supply circuit.
The increasing demand for higher performance power supply circuits has resulted in the continued development of voltage regulator devices. Many low voltage applications are now requiring the use of low dropout (LDO) regulators, such as for use in cellular phones, pagers, laptops, camera recorders and other mobile battery operated devices. These portable electronics applications typically require low voltage and quiescent current flow to facilitate increased battery efficiency and longevity. The alternative to low drop-out regulators are switching regulators which operate as dc-dc converters. Switching regulators, though similar in function, are not preferred to low drop-out regulators in many applications because switching regulators are inherently more complex and costly, i.e., switching regulators can have higher cost, as well as increased complexity and output noise than low drop-out regulators.
Low drop-out regulators generally provide a well-specified and stable dc voltage whose input to output voltage difference is low. Low drop-out regulators typically have an error amplifier in series with a pass device, e.g., a power transistor, which is connected in series between the input and the output terminals of the low drop-out regulator. The error amplifier is configured to drive the pass device, which can then drive an output load. The operation of the low drop-out regulator is based on a control loop, which includes the feeding back of an amplified error signal used to control the output current flow of the power transistor driving the output load. The drop-out voltage of the low drop-out regulator is defined as the value of the input/output differential voltage that the control loop stops regulating. Low drop-out regulator 100 also typically requires large output capacitors that are required to have a low electrical series resistance (ESR). However, such capacitors tend to require large circuit board area, and thus are highly responsible for the overall cost of the low drop-out regulator.
Such a low drop-out regulator generally has two inherent characteristics including the magnitude of the input voltage being greater than the respective output voltage, and the output impedance being low so as to yield good performance. Low drop-out regulators can also typically be categorized as either low power or high power. Low power low drop-out regulators generally have a maximum output current of less than 1 A, and are used mainly by the above portable applications. On the other hand, high power low drop-out regulators can yield currents that are equal to or greater than 1 A at the output, which can be demanded by many automotive and industrial applications.
With reference to FIG. 1, a schematic diagram of a conventional low drop-out regulator 100 is illustrated. Low drop-out regulator 100 includes an error amplifier 102 and a pass device 104 configured in a feedback arrangement. Error amplifier 102 is configured to drive a low current during DC conditions, and a high current, e.g., 1 mA, under high slew or transient conditions. Error amplifier typically includes a class AB-type amplifier device. Error amplifier 102 has a positive input connected to a reference voltage VREF, and powered by an input supply voltage VIN. Reference voltage VREF, which usually includes a zener diode for high voltage applications or a bandgap reference for low voltage and high accuracy applications, is configured to provide a stable dc bias voltage with limited current driving capabilities.
Pass device 104 comprises a power transistor device MP configured for driving an output current IOUT to a load device. Pass device 104 has a control terminal suitably coupled to the output of error amplifier 102 and can include various configurations, such as NPN follower, NMOS follower, or common emitter PNP or common source PMOS transistors. Bipolar devices are generally used for applications requiring higher output currents and are capable of generating higher quiescent currents, while MOS devices are generally used for applications requiring minimized quiescent current. For bipolar devices, the beta xcex2 is defined as the ratio of the collector current to base current. This base current can be large and is often driven into ground, i.e., the ground current is increased considerably. For a low drop-out regulator, beta is also a measure of the efficiency, i.e., the ratio of the output current IOUT to the ground current. Because the bipolar device is considered a current gain device, the beta xcex2 can be quite low, ranging approximately from 100 to 1000. Thus, for every milliamp of current delivered at the output IOUT, 1 xcexcA to 10 xcexcA would be delivered to ground, i.e., for 100 mA of output current, between 100 xcexcA and 1000 xcexcA of ground current are realized, resulting in poor efficiency for such bipolar devices.
Accordingly, CMOS transistor pass devices are usually the best overall configuration for optimizing efficiency. In the example of FIG. 1, pass device 104 includes a PMOS transistor device, which typically requires very low DC current under full load conditions. Pass device 104 receives at a control terminal, e.g., gate terminal, an amplified error signal from error amplifier 102 configured to control the output current flow of pass device 104 when driving the output load at an output terminal VOUT. Pass device 104 is configured to feed back the error signal to error amplifier 102.
Pass device 104 also introduces large, parasitic capacitances C1 and C2 to low drop-out regulator 100. The large capacitances, for example 100 pF or more, can limit the capability of error amplifier 102, since the capacitances require high current during a fast transition. For example, when designing devices configured to respond rapidly to changes in the output load, pass device 104 requires a large amount of current since parasitic capacitances C1 and C2 must be charged and discharged. Thus, in transient conditions, milliamps of current during microsecond periods must be supplied by error amplifier 102 just to charge parasitic capacitances C1 and C2.
In addition to the requirement for higher current during transient conditions, other constraints are present on error amplifier 102. For example, as currently available power systems are demanding the use of less operating supply voltage VIN, such as an operating voltage of 1.8 volts, low drop-out regulator has to operate within one gate-source voltage VGS, or approximately within a threshold voltage VT of the pass device plus an extra voltage xcex94. Thus for a single gate-source voltage VGS topology, to turn on pass device 104 with a threshold voltage VT of 0.7 to 1.2 volts, error amplifier 102 must provide at least that voltage plus the extra voltage xcex94, all within the limited headroom of 1.8 volts.
Another constraint on error amplifier 102 is the need to control the offset of the low drop-out regulator. In other words, not only does error amplifier 102 need to comprise a class AB device that can drive a lot of output current, while also providing a low quiescent current during low voltages, error amplifier 102 also needs to minimize the offset contribution.
Yet another constraint of error amplifier 102 is the compensation requirement. As discussed above, pass device 104 includes large parasitic capacitances, thus often requiring the implementation of a buffer, or a gm boost, to isolate the high output resistance of the gain stage of error amplifier 102 from the high load capacitance of pass device 104. For example, with reference to FIG. 2, a low drop-out regulator 200 implementing a buffer 206 between the output of an error amplifier 202 and a pass device 204 is illustrated. Buffer 206 is configured to receive the output current from error amplifier 202 and drive the gate of pass device 204. The output from buffer 206 can be mirrored back through a complex, current mirror circuit including transistors M1 through M5 to compensate error amplifier 202. Other schemes can further incorporate an additional operational amplifier at the output of the pass device to sense the output current. However, adding such additional components can create stability problems. In addition, low drop-out regulator 200 generally has a lower efficiency due to a higher ground current, i.e., the current mirror circuit including transistors M1 through M5 tends to drive current to ground.
In some applications, with reference to a low drop-out regulator 300 illustrated in FIG. 3, a buffer 306 can includes a bipolar follower configuration, which is biased in class A operation. However, in either compensation scheme, current is being taken from the supply and driven into ground, i.e., the ground current is increased considerably, resulting in reduced efficiency.
In addition, for bipolar buffer configurations, headroom limitations are often readily apparent. For example, to buffer error amplifier 302 and to drive pass device 304, NPN follower device 306 needs to be at least a base-emitter voltage VBE above the drive voltage, i.e., level shifting of the voltage at the gate of pass device 304 is necessary. Thus, for a drive voltage of 0.8 volt needed to drive the gate of pass device 304, and with a base-emitter voltage VBE of 0.8 to 1.0 volt, very little headroom is available for lower voltage power supply circuits, such as those with supply voltages VIN of 1.8 volts. As a result, control of pass device 304 can be difficult.
Accordingly, a need exists for a low drop-out regulator that provides higher performance and efficiency, and that can overcome the various problems with prior art low drop-out regulators.
The method and circuit according to the present invention addresses many of the shortcomings of the prior art. In accordance with various aspects of the present invention, a low drop-out regulator is configured to provide high output current with a fast response during transient conditions, while also maintaining low quiescent current under DC conditions.
In accordance with an exemplary embodiment, an exemplary low drop-out regulator comprises an error amplifier, a current feedback amplifier, and a pass device. The low drop-out regulator includes a composite amplifier feedback configuration, with the current feedback amplifier being decoupled from the overall composite feedback configuration and configured to provide effective compensation. As a result, the current feedback amplifier can be configured to operate with low current supplied from the error amplifier and to drive the control terminal of the pass device with sufficiently high current as demanded by a load device.
In accordance with another aspect of the present invention, the current feedback amplifier can be configured to permit the voltage at the control terminal of the pass device to operate from rail-to-rail. In accordance with an exemplary embodiment, instead of providing the feedback and reference signals into the high impedance control terminals of a pair of input devices, the current feedback amplifier is configured with a feedback and/or reference signal being provided to the low impedance input terminals of a pair of input devices. As a result, current is forced through the pair of input devices and can be suitably utilized to supply the low drop-out regulator with the ability to provide rail-to-rail output drive capabilities from an output device of the current feedback amplifier to the pass device.
In accordance with another aspect of the present invention, the gain and offset of the low drop-out regulator can be provided by the error amplifier, without the requirement to drive a high amount of current to the current feedback amplifier.